The present invention relates to a measuring system for preparing working gauges with a measurement precision of 1 .mu.m.
A working gauge, also called a master, is an e.g. block shaped body, which is machined to show a desired exact distance between two surface areas, whereby it is usable as a comparison gauge in connection with high precision working of various production parts. When the gauge has a correct measurement, it will be sufficient to carry out a comparative measuring of the production part, e.g. with the use of a stand having a sufficiently accurate dial gauge. Often it is necessary, e.g. during a day of production, to carry out several measurings of the gauge for a steady checking up of the other measuring equipment, and this implies a certain wear of the gauge, even if it is made of hard steel. Therefore, all according to the frequency of use and other conditions of use, the gauge block should be checked up from time to time by an exact and absolute measuring, which requires an expensive measuring apparatus. In large enterprises having such an apparatus, e.g. a three-coordinate measuring machine, there are often queues before the respective measuring departments, and the single measurings are time-consuming.
If or when the absolute measurement of a gauge has changed, the gauge may still be fully usable, at least by small deviations, when it is marked--preferably by engraving--with an indication of its deviation from its nominal measurement, e.g. "-4 .mu.m". In use it is hereby possible to effect the required corrections by an exact dial gauge or a similar device which, without being too costly, can accurately measure inside a narrow measuring range of e.g. 10 mm, while the total measuring range employed may be up to some 20-50 cm. It has been found, however, that the users nevertheless sometimes forget to carry out the required corrections, and as well known this may give rise to an expensive waste or to serious machinery break downs later on.
Instead of spending time on preparing a master gauge with associated absolute control measurings during the preparation thereof, it may be chosen to assemble a master gauge from standard calibrated gauge blocks, which should need no control measuring. However, these blocks are quite expensive, and it has been found that the blocks are not as accurate as commonly believed. In this connection, the heat from manual handling of a short duration may be sufficient to cause a change of several .mu.ms, and moreover even these blocks will be worn if they are used as practical measuring implements and not only as adjustment standards in protected surroundings in specialized laboratories, so-called measuring rooms. Worn blocks may well be calibrated, but only at very high costs and with a long waiting time, since the production and the calibration of gauge blocks is a specialty practiced by only very few manufacurers.
In this connection, it should be mentioned that accurate length measurement standards for industrial use are found almost exclusively as the gauge blocks, which may be placed together to form any measurement of addition under controlled circumstances, e.g. by a room temperature of 20.degree. C. Otherwise the users have to provide their own gauges or masters based on the use of accurate measuring instruments, which should of course also work under controlled conditions. The standard blocks are provided with entirely smooth and exactly plan parallel surfaces which by disposing the blocks together may cause the blocks to stick together already by the associated complete displacement of air.
From a measuring technical point of view, it is very important that the opposed surfaces are really completely plan parallel, as a measurement difference of e.g. one half of one thousandth of a millimeter between the opposite ends of the blocks might increase by the laying together of five-six blocks. Also, the possible high accuracy of inexpensive short distance measuring instruments such as dial gauges and micrometer screws normally is conditioned by these instruments co-acting with a surface which is exactly perpendicular to the moving direction of the actuator pin of the instrument, as the actuator pin could otherwise be subjected to disturbing lateral force components.
With the use of standard gauge blocks, it is possible to build up a master gauge having exactly the required measurement (a so-called zero master), as well as to build up a master gauge having the required measurement only approximately, whereafter the final measurement may be determined with the additional use of an exact short distance measuring device. However, the gauge blocks should still be checked by a control measuring from time to time, as an operator could otherwise, in practice, carry on producing faulty objects with full confidence of the required tolerances being observed, even when given correction values are duly taken into account.
It is the purpose of the invention to provide a measuring element system which may widely simplify and ensure the manufacturing of articles with correct length measurements of high precision.
According to the invention, for the provision of a working gauge or master use is made of one or more length variable and fixable master units, which, in a measuring apparatus and not under laboratory conditions, are adjusted and fixed at a length dimension corresponding to the length of a calibrated gauge as measured in the same or a corresponding measuring apparatus, whereby the calibrated gauge includes laid together, conventional gauge blocks. The measuring apparatus includes an accurate short distance measuring device such as a dial gauge, which enables a measuring out with the required high accuracy over a short distance of e.g. 20-30 mm. By this construction, it is possible to build up all possible master dimensions based on a rather small number of precalibrated gauges, and the master itself may correspondingly include a small number of standard units. As the length dimension of the master unit is adjustable, it is possible to check and correct the master by way of a simple adjustment with the use of the measuring apparatus and the calibrated gauge. Thus, if a check reveals a changed dimension of the master, the correct dimension may be rapidly and easily reestablished, such that, with the use of the master, it will not be necessary to calculate with measurement corrections, i.e. the master will steadily be a "zero master", whereby the risk of tolerance errors of the produced articles is considerably reduced.
To all this it is of course of utmost importance or a superior condition that the adjustable master units be practically suitable and dimensionally stable, and it is also important that the precalibrated gauge blocks should be relatively inexpensive and dimensionally stable.
A significant feature of the present invention is that the invention provides for a new type of a "gauge block" which is adjustable and usable with high accuracy, and which is usable both for easily readjustable working gauges or masters and for preadjusted gauges for absolute measurements. An adjustable conventional gauge block can hardly be imagined, but in connection with the invention, it has been realized that for the relevant use of the calibrated gauges, there is no practical need of a block having broad and plan parallel surfaces, since it is perfectly possible and even advantageous to make use of a block member having but a single broad support surface and an opposite measuring surface which may have a very small area or even be almost point shaped; in both cases the measuring surface may consist of an end portion of a pin, which is fixably length displaceably received in a bore in a block member shaped with the broad support surface.
In, for example, U.S. Pat. No. 4,219,934, a measuring arrangement is proposed, which comprises the use of adjustable gauge elements and an associated measuring apparatus. These proposed gauge elements include a base block, in which there is provided a screw device, which is manually operable for adjusting a pin projecting from the base block. A clamp enables the pin to be fixed in some desired position, whereby the gauge measurement will be the distance between the free end of the pin and the opposed end surface of the base block. Such master elements with their associated threaded systems are very expensive to manufacture, and since the threaded systems occupy a considerable space in the direction of the gauge measurement, the elements have a rather large minimum thickness. Furthermore, the measuring apparatus used for the adjustment of the gauges in this proposed arrangement is unable to satisfy the tolerance requirements in the .mu.m range, because the apparatus largely corresponds to a usual slide gauge with a nonius system, which is entirely insufficient. Thus, the proposed arrangement will not in any way obviate the use of the gauge blocks, when really precise measurings or adjustments are to be made, and the proposed construction is rather one of many examples of the unlucky fact that the operators may well consider extremely small relative measurement variations in the outermost end of the measuring system, yet without any correspondingly accurate reference to the absolute measurement. In practice just this circumstance is responsible for many machine break downs, even though a mechanic has conscientiously observed the actual tolerance requirements based on a working gauge, the measurement of which may deviate considerably from the desired absolute measurement.
According to the invention, the block member is preferably a circular cylindrical body, in which the pin is received with slide fit in a central axial bore, with the pin being fixable by a radial clamp screw in the cylindrical body. This block design is well usable both for adjustable masters and for fixed gauges for absolute measurements, where the latter are calibrated by existing high precision gauges or highly precise measuring machines. The calibration may be effected by displacement and fixation of the pin, i.e. without machining the material, whereby a high calibration precision is obtainable in a simple and inexpensive with the outer end of the pin being preshaped with a desired hardness and shape. The gauges for absolute measurements are produceable at essentially reduced costs compared with the conventional gauge blocks.
The general applicability of these novel standard gauges may, possibly, be somewhat limited compared with the conventional gauge blocks, but what is here in focus is the use of precise gauges as a reference for following measurings in the measuring apparatus for working gauges, and in this particular relation the standard gauges of the invention will be not only less expensive, but improved as compared with the conventional blocks. The circular base surface of the block members, preferably constituted by an outermost annular area at the bottom side of the block member, will, when reasonably dimensioned, e.g. with an outer diameter of 5 cm, provide for a measuring-technically better rest against a carrier surface as compared with a side of a conventional gauge block which corresponds approximately to a narrow side of a small match box; the ring circular support surface will, principally, be uniformly three-point-supported no matter how it is turned, which does not apply to a rectangular surface.
Moreover, the gauges according to the invention will show the advantage that compared with conventional gauge blocks, they are less liable to undergo measurement changes resulting from short temperature changes of the outer surface thereof, e.g. when being manually touched, since the central pin is to some degree heat insulated from the surrounding block body.
A further significant feature of the invention is that the measuring apparatus is designed to be able to measure the axial height of a central pin of a calibrated standard gauge and an adjustable working gauge, respectively, of the type provided by the invention, since it is hereby possible to obtain well defined measurings with the aid of a short distance measuring instrument of a type operating directly in or symmetrically about the associated measuring axis, i.e. without the measuring itself (as with a slide gauge) taking place in a line offset from the real measuring line with associated uncertainty factors.
For carrying out the desired adjustment of the gauge according to the invention it is an obvious problem that it is difficult to effect a direct pin displacement with an accuracy of the magnitude 1 .mu.m, and it is important, therefore, that the measuring apparatus according to the invention, as well as the employed masters or working gauge units, are designed such that the measuring apparatus has an operation section including an actuator rod, which is operable, through a gear coupling, to carry out a slow displacement which is transferred to the central pin of the master unit, whereby this pin is easily adjusted and then fixable with the desired very high precision.
In connection with the invention, it is a special problem that the temporarily fixed pin of the master unit should be fixed sufficiently hard to make sure that it will not be displaced as much as a single .mu.m in response to forces applied from the outside. This, of course, is achievable by a very hard clamping of the pin, but it has proved difficult to indicate some standard for such a "sufficiently hard clamping", whereas the clamping action itself may well give rise to a length increase of the pin. As disclosed in more detail below the invention, therefore, comprises a special clamping system, whereby it is achieved that the central pin will either not at all be clamped or will be fully clamped with a sufficient stability of measurement, such that the operator can easily decide whether the clamping is effective or not.